Many liquid or pourable food products, such as fruit juice, UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are typically sold in packages made of sterilized packaging material.
A typical example is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is formed by creasing and sealing laminated strip packaging material.
The packaging material has a multilayer structure comprising a base layer, e.g. of paper, covered on both sides with layers of heat-seal plastic material, e.g. polyethylene. In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material may also comprise a layer of gas and light-barrier material, e.g. an aluminium foil, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package ultimately contacting the food product.
Packages of this sort are normally produced on fully automated packaging machines, on which a continuous tube is formed from the web-fed packaging material. The web of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is conveniently removed from the surfaces of the packaging material, e.g. evaporated by heating. The web thus sterilized is then maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a tube, which is fed vertically with the food product.
In order to complete the forming operations, the tube is filled with the sterilized or sterile-processed food product, and is sealed and subsequently cut along equally spaced cross sections.
More precisely, the tube is sealed longitudinally and transversally to its own axis. From this sealing operation, pillow packs are obtained, which have a longitudinal seal and respective top and bottom transversal seals.
Each pack also comprises, for each top and bottom end portion, an elongated substantially rectangular fin formed by respective sealing bands; and a pair of substantially triangular flaps projecting from opposite sides of relative end portion and defined by respective trapezoidal walls.
The end portions are pressed towards each other by the folding unit to form flat opposite end walls of the pack, while at the same time folding the flaps of the top portion onto respective lateral walls of the main portion and the flaps of the bottom portion onto the bottom sealing line.
Packaging machines of the above type are known, on which the packs are turned into folded packages by automatic folding units.
Folding units are known, for example from EP-B-0887261 in the name of the same Applicant, which substantially comprise:                a chain conveyor for feeding packs continuously along a forming path from a supply station to an output station;        a number of folding devices arranged in fixed positions relative to the forming path and cooperating with packs to perform relative folding operations thereon;        a heating device acting on respective triangular flaps of each pack to be folded, to melt and seal them onto respective walls of the pack; and        a pressing device cooperating with each pack to hold the triangular flaps in the respective folded positions as these portions cool.        
The step of transferring and feeding packs to the inlet station of the folding unit is critical for proper operation of the folding unit.
In practice, the packs are typically formed and sealed with their longitudinal axis arranged vertically. Subsequently, the newly formed packs are received by a feeding conveyor, which drives the packs to the inlet station of the folding unit.
The feeding conveyor comprises, in turn: a first endless belt and a second endless belt, and a plurality of push members carried by the first endless belt and the second endless belt.
The first endless belt and the second endless belts are wound onto respective pulleys having, in use, corresponding vertical axes and define therebetween a passage, horizontal, in use, for the packs.
Still more precisely, each pack is advanced within the passage by a respective push member of the first endless belt and a respective push member of the second endless belt.
Each push member comprises, in turn, a cooperating surface, which contacts a relative pack to push it towards an outlet station of the feeding conveyor.
In order to ensure very smooth motion conditions for the pillow packs being fed to the receiving station of a folding unit, it has been proposed, in the European patent EP 2586715 in the name of the same Applicant, to shape the profile of the cooperating surface of each push members as an involute.
In this way, the packs, when are advanced towards the folding unit, are subjected to a substantially constant and homogeneous thrust, especially when they are released from the respective push members.
Even performing well, the above-identified solution leaves room for an improvement.
In particular, when the push members move in a substantially vertical plane, there is the risk that the involute-shaped cooperating surface of push members slides below the pillow packs, substantially behaving as respective wedges.
There is therefore the risk that the speed of the packs varies, when the packs are released by the push members, thus generating the risk of a suboptimal shaping of the corresponding folded packages.
A need is felt within the industry to feed the packs to the folding unit according an alternative way, which can meet the above-identified requirement.